2.8 Curve bevel and taper scaling issues
Partial answer, in order to scale a beveled curve without scaling the beveled section/object,


Scale your curve,
Apply Scale then uncheck the Apply Properties of the Apply Object Transform




As far as Taper goes, it only influences the bevel profile in the X/Y space. I.e. if the taper curve has some Z dimension, it will be ignored.

The full X extent of the taper object applies proportionally (i.e. normalized to) to the full length of the tapered curve. In other words, if the taper object extends between x=-1.0 to x=+1.0 (relative to its origin): 


the Y coordinate at x=-1.0 will define the bevel profile width at the start of the parent/tapered curve and
the Y coordinate at x=+1.0 will define the bevel profile width at the end of the parent/tapered curve


The Y scaling is relative to 1m in the Y axis from the origin. Also a side effect is that scaling the taper object in the X axis will not change anything since the normalization to the parent curve length occurs.



If you want to control the exact dimension of the taper effect on the bevel profile in relation to your bevel object,  take the bevel dimension and calculate the scale factor required at the different taper positions along the parent curve and make sure that the Y coordinate of the corresponding taper control point has the right scale factor to obtain the desired value.

For example: with a bevel circle of 3mm radius if you want to obtain a bevel width of 5mm in the middle of the parent curve, make sure that the control point in the X center of the taper curve has a Y value of 5/3m from its origin in the Y axis